regression: dictionary key no autocomplete fix

Fix: #38998
(cherry picked from commit 13c372b5228f6f17a23aadb8fc59230c5a08c176)

modules/gdscript/gdscript_editor.cpp

@@ -2098,7 +2098,11 @@ static void _find_identifiers_in_base(const GDScriptCompletionContext &p_context
 
 				if (!p_only_functions) {
 					List<PropertyInfo> members;
-					tmp.get_property_list(&members);
+					if (p_base.value.get_type() != Variant::NIL) {
+						p_base.value.get_property_list(&members);
+					} else {
+						tmp.get_property_list(&members);
+					}
 
 					for (List<PropertyInfo>::Element *E = members.front(); E; E = E->next()) {
 						if (String(E->get().name).find("/") == -1) {